This invention relates to systems and methods for ion beam scanning of semiconductor wafers and other workpieces and, more particularly, to systems and methods for high efficiency ion beam scanning wherein overscan is limited and dose uniformity is achieved.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. Most ion implanters use an ion beam that is much smaller than the wafer in both dimensions and distribute the dose from the ion beam across the wafer by scanning the beam electronically, moving the wafer mechanically, or by a combination of beam scanning and wafer movement. Ion implanters which utilize a combination of electronic beam scanning and mechanical wafer movement are disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al and U.S. Pat. No. 4,980,562 issued Dec. 25, 1990 to Berrian et al. These patents describe techniques for scanning and dosimetry control in such systems. The beam efficiency in the disclosed systems is limited by the fact that the ion beam is scanned in a square pattern, whereas the wafer is round. Inefficiency in scanning arises from the fact that the ion beam is directed at the wafer during only a portion of the scan time. Efficiency is also limited by the need to overscan the wafer to reach the Faraday beam sensor and to make the dose uniform at the edges of the wafer.
Another drawback of the prior art scan techniques is that the ion beam may be left at a fixed position on one side of the wafer between electronic scans for variable amounts of time to allow the dose to be varied by changing the amount of time that the beam spends on the wafer. This can be a problem because contaminant beams may be present in addition to the main beam. The contaminant beams follow a different path through the ion optics. If the path of one of the contaminant beams falls on the wafer when the beam is at its fixed position, a concentration of dose is produced by the contaminant beam at that spot on the wafer. This problem can occur in ion implanters that use bipolar deflection to scan the ion beam.
U.S. Pat. No. 4,633,138 issued Dec. 30, 1986 to Tokoguchi et al discloses an ion implanter wherein the width of a beam scan is controlled to approximate the shape of the wafer in response to a width sensor. The wafer speed is controlled to compensate for dose variations which result from different sweep widths. U.S. Pat. No. 4,260,897 issued Apr. 7, 1981 to Bakker et al discloses a technique for ion implantation wherein the beam sweep is controlled to match the shape of the target. Curved sensors on each side of the target detect the ion beam and initiate reversal of the sweep. U.S. Pat. No. 4,421,988 issued Dec. 20, 1983 to Robertson et al discloses a technique for ion beam scanning wherein the scan width is matched to the width of the target wafer by means of a predetermined sequence of scan times.
All of the known prior art scanning techniques have had one or more drawbacks, including limited efficiency and limited applicability. For example, the techniques may be limited to ion implanters which employ two-dimensional electrostatic ion beam scanning. Accordingly, there is a need for improved methods and apparatus for high efficiency scanning in ion implanters.
The present invention relates to ion implanters wherein the ion beam is scanned electronically in one direction, typically horizontally, and the wafer is translated mechanically in a second direction, typically vertically, to distribute the ion beam over the wafer surface. The speed of mechanical translation is varied so that the wafer is travelling more slowly as the beam is scanned across the center of the wafer where it is wider in the electronic scan direction and faster at the top and bottom where the wafer is narrower in the electronic scan direction. The beam scan width is varied in coordination with wafer translation to maintain uniform dose on the wafer while avoiding wasting the ion beam by scanning the beam off the wafer. Preferably, the electronic scan is such that the ion beam is moving at all times so that contaminants are scanned and do not concentrate at one point on the wafer.
According to a first aspect of the invention, a method is provided for ion implantation of a workpiece. The method comprises the steps of generating an ion beam, scanning the ion beam across a workpiece in a first direction in response to a scan waveform that defines a beam scan width, translating the workpiece in a second direction at a translation velocity relative to the ion beam so that the ion beam is distributed over the workpiece, and controlling the translation velocity and the beam scan width to limit the time that the ion beam is off the workpiece.
According to another aspect of the invention, ion implantation apparatus is provided. The ion implantation apparatus comprises an ion beam generator for generating an ion beam, a scanner for scanning the ion beam across a workpiece in a first direction in response to a scan waveform that defines a beam scan width, a mechanical translator for translating the workpiece in a second direction at a translation velocity relative to the ion beam so that the ion beam is distributed over the workpiece, and a controller for controlling the translation velocity and the beam scan width to limit the time that the ion beam is off the workpiece.